This invention relates to centrifugal or rotary fuel pumps. More particularly, it relates to fuel pumps for aircraft gas turbines.
Fuel pumps for aircraft gas turbines traditionally have been positive displacement types, most often gear pumps. These pumps are normally driven at a fixed ratio to engine speed. Positive displacement pumps have two major advantages over centrifugal pumps, which make them attractive for use as gas-turbine fuel pumps. First, this type of pump exhibits good dry suction characteristics, eliminating the necessity of boost pumps for priming. Secondly, positive displacement pumps provide sufficient pressure over a wide range of engine speeds. The pump sizing point typically is the flow and pressure needed for engine light-off at cranking speeds. This sizing criteria, however, results in excessive fuel delivery at higher engine speeds and altitudes, since the pump speed is tied to engine speed. This overflow requires a fuel bypass loop. Bypass and recirculation of fuel, though, results in significant fuel heating.
With the latest fuel efficient engine designs, excessive fuel heating becomes a serious problem. Reduced engine fuel consumption is accompanied by increased engine and lubrication system temperatures. With lower fuel temperatures, fuel/oil heat exchangers are capable of removing more heat from the lubrication oil, reducing the thermal load and therefore size of the air/oil heat exchangers. Associated with the air/oil heat exchangers is a significant drag and weight penalty to the aircraft.
Centrifugal pumping systems offer reduced fuel temperature rise when compared to a positive displacement pump. These rotary pumps can simply be throttled to eliminate excess fuel delivery, so no flow bypass loop is needed. The largest heat savings is at low flows and high engine speeds, where bypass flow in a positive displacement system is at a maximum. Other advantages include increased reliability and decreased weight. Centrifugal pumps are rarely used as aircraft fuel pumps, however, because of their inability to supply adequate pressure at low speeds and their poor dry suction characteristics.
Accordingly, it is the principal object of the present invention to provide a centrifugal fuel pump, for aircraft gas turbines, that overcomes the problems of the prior art.
It is a more specific object to provide an improved centrifugal fuel pump, as above, with a unique starting stage that supplies adequate pressure during engine start-up, as well as good dry suction characteristics.
It is another specific object to provide such a fuel pump with two different styled impellers on opposite sides of the same rotor--namely, a regenerative/liquid-ring pump to be used for start-up and low speeds, and a high-speed centrifugal pump (on the opposite side) to operate during the aircraft's normal operation.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.